Liquid crystals are used primarily as dielectrics in display devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a "twisted nematic" structure, STN cells ("super twisted nematic"), SBE cells ("super birefringence effect") and OMI cells ("optical mode interference"). For displays having a high content of information, actively controlled cells, e.g. TFT cells ("thin film transistor"), have in particular recently become important in addition to passively controlled, multiplexed cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical, photochemical and thermal stability and a good stability towards electric fields. Further, they should have a suitable mesophase over a range which is as broad as possible (for example, a nematic or a cholesteric phase for the cells referred to above), but in spite of a sufficiently low viscosity should permit short response times, low threshold potentials and a high contrast in the cells. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the field of application and type of cell. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy which is as high as possible and at the same time should have a conductivity which is as low as possible. The latter property is of particular importance primarily for actively addressed liquid crystal displays having a high density of information, e.g. TFT cells. Unfortunately, however, components having a high dielectric anisotropy mainly lead to an increased conductivity in mixtures because of their improved capacity to dissolve ionic impurities. Accordingly, components which are distinguished by a dielectric anisotropy which is as high as possible with simultaneously a conductivity which is as low as possible are sought after.